1. Field of the Invention
Ceramic aerogels are among the most highly porous and lowest density materials. Their high porosity means that 95% or greater of the total bulk volume of a ceramic aerogel is occupied by empty space (or air), producing excellent thermal as well as sound insulating qualities. In addition, their high specific surface area (e.g. on the order of 600-1000 m2/g) should make them well suited for numerous applications, including as adsorbent beds for chemical separations, as catalyst supports, as platforms for solid state sensors, etc. Unfortunately, conventional ceramic aerogels are physically and hydrolytically very unstable and brittle. Their macro-structure can be completely destroyed by very minor mechanical loads or vibrations, or by exposure to moisture. In addition, over time, these materials tend to produce fine particles (dusting) even under no load. Consequently, there has been little interest in ceramic aerogels for the above-mentioned as well as other applications, despite their excellent properties, simply because they are not strong enough to withstand even minor or incidental mechanical stresses likely to be experienced in practical applications. To date, such aerogels have been used almost exclusively in applications where they will experience no or almost no mechanical loading.
2. Description of Related Art
U.S. Patent Application Publication No. 2004/0132846, the contents of which are incorporated herein by reference, describes an improvement wherein a diisocyanate is reacted with the hydroxyl groups prevalent on the surfaces of secondary (φ5-10 nm) particles of a silica aerogel to provide a carbamate (urethane) linkage. Additional diisocyanate monomers are further polymerized to produce a network of polyurea chains between the carbamate linkages of respective pairs of hydroxyl groups present on the secondary particles, resulting in a conformal polyurea/polyurethane coating over the silica backbone. The resulting structure was found to have only modestly greater density than the native silica gel (2-3 times greater), but more than two orders of magnitude greater mechanical strength, measured as the ultimate strength at break for comparably dimensioned monoliths. The present invention provides further improvements beyond what is disclosed in the 2004/0132846 publication.